The superior colliculus of the cat exhibits neurons which respond to sensory stiumli and neurons which respond before movements of the head and eyes. The primary goal of this research is to analyze the connectional relationships of these cell types using a combination of light and electron microscopic techniques. The main approach is to characterize with the electron microscope inputs to superior colliculus cells in the superficial or intermediate grey layer which have been identified either physiologically and then injected intracellularly with horseradish peroxidase (HRP) or anatomically, by diffusely backfilling the soma and dendrites with horseradish peroxidase injected into the axons or terminal arborizations of these cells. In the same animals, potential inputs to these HRP-filled cells will be labelled either autoradiographically or by anterograde degeneration. In the electron microscope, we can determine whether axon terminals labelled either autoradiographically or by degeneration contact the dendrites or somas of neurons filled with the electron-dense horseradish peroxidase reaction product. The results should shed light on how various inputs converge on specific, identified cell types in the superior colliculus.